1. Field of the Invention
The present invention relates generally to methods of fabricating for electrically connecting electronic devices. More particularly, the invention relates to an improved method for fabricating anisotropic electrically conductive materials which can provide an electrical interface between devices placed on either side thereof.
Over the past ten years, electrically conductive elastomers have found increasing use as interface connectors between electronic devices, serving as an alternative for traditional solder and socket connections. Elastomeric conductors can take a variety of forms, but generally must provide for anisotropic electrical conduction. Anisotropic conduction means that the electrical resistance measured in one direction through the material will differ from that measured in another direction. Generally, the elastomeric conductors of the prior art have been materials which provide for high resistance in at least one of the orthogonal directions of the material, while providing low resistance in the remaining one or two directions. In this way, a single piece or sheet of material can provide for multiple connections so long as the connector terminals on the devices to be connected are properly aligned.
2. Description of the Prior Art
The anisotropic elastomeric conductors of the prior art generally consist of an electrically conductive material dispersed or arranged in an electrically insulating material. In one form, alternate sheets of conductive and non-conductive materials are layered to form a block, and individual connector pieces can be cut from the block in a direction perpendicular to the interface of the layers. Connector pieces embodying such layered connectors have been sold under the trade name "Zebra" by Tecknit, Cranford, N.J., and the trade name "Stax" by PCK Elastomerics, Inc., Hatboro, Pa. Such connectors are discussed generally in Buchoff, "Surface Mounting of Components with Elastomeric Connectors," Electri-Onics, June, 1983; Buchoff, "Elastomeric Connections for Test & Burn-In," Microelectronics Manufacturing and Testing, October, 1980; Anon., "Conductive Elastomeric Connectors Offer New Packaging Design Potential for Single Contacts or Complete Connection Systems," Insulation/Circuits, February, 1975; and Anon., "Conductive Elastomers Make Bid to Take Over Interconnections," Product Engineering, December 1974. While useful under a number of circumstances, such layered anisotropic elastomeric conductors provide electrical conductivity in two orthogonal directions, providing insulation only in the third orthogonal direction. Thus, the layered anisotropic elastomeric conductors are unsuitable for providing surface interface connections where a two-dimensional array of connector terminals on one surface is to be connected to a similar two-dimensional array of connectors on a second surface. Such a situation requires anisotropic elastomeric conductor which provides for conductivity in one direction only.
At least two manufacturers provide anisotropic elastomeric conductors which allow for conduction in one direction only. Tecknit, Cranford, N.J., manufactures a line of connectors under the trade name "Conmet." The Conmet connectors comprise elastomeric elements having two parallel rows of electrically conductive wires embedded therein. The wires are all parallel, and electrical connections may be made by sandwiching the connector between two surfaces so that good contact is established. The Conmet connector is for connecting circuit boards together, as well as connecting chip carriers and the like to printed circuit boards. The matrix is silicon rubber.
A second anisotropic elastomeric conductor which conducts in one direction only is manufactured by Shin-Etsu Polymer Company, Ltd., Japan, and described in U.S. Pat. Nos. 4,252,391; 4,252,990; 4,210,895; and 4,199,637. Referring in particular to U.S. Pat. No. 4,252,391, a pressure-sensitive electroconductive composite sheet is prepared by dispersing a plurality of electrically conductive fibers into an elastomeric matrix, such as silicone rubber. The combination of the rubber matrix and the conductive fibers are mixed under sheer conditions which break the fibers into lengths generally between 20 to 80% of the thickness of the sheet which is to be prepared. The fibers are then aligned parallel to one another by subjecting the mixture to a sheer deformation event, such as pumping or extruding. The composite mixture is then hardened, and sheets prepared by slicing from the hardened structure. The electrically conductive fibers do not extend the entire thickness of the resulting sheets, and electrical contact is made through the sheet only by applying pressure.
Although useful, the anisotropic elastomeric conductors of the prior art are generally difficult and expensive to manufacture. Particularly in the case of the elastomeric conductors having a plurality of conductive fibers, it is difficult to control the density of fibers at a particular location in the matrix, which problem is exacerbated when the density of the conductive fibers is very high.
For these reasons, it would be desirable to provide alternate methods for fabricating anisotropic elastomeric conductors which provide for conductivity in one direction only. In particular, it would be desirable to provide a method for preparing such elastomeric conductors having individual conductive fibers present in an elastomeric matrix in a precisely controlled uniform pattern.